Coating of paper



June 8, 1943.

F. G. RAWLING- COATING OF PAPER Filed March 13, 1940 SUPER/"P0550 M4722F was CQATYNG COLOE SUPPLY PAIER WEB SY/VEEES/S OCCl/k HEEE mvEN-roRfikA/vas GTE/mans 'f'ToRNEY Patented June 8, 1943 UNITED STATES PATENToFFieE COATING OF PAPER Francis G. Bawling, Piedmont, W. Va., assignorto West Virginia Pulp and Paper Company, New York, N. Y., a corporationof Delaware Application March 13, 1940, Serial No. 323,680

2 Claims. 117- 65) My present invention relates to the art of coatingpaper and the like. Compositions for the coating of paper commonlyconsist of a finely divided mineral pigment or filler, e. g., clay,chalk, calcium sulflte, titanium oxide, satin white, together with anadhesive such as casein or other protein, cooked-starch, the whole beingground and suspended in water. Such compositions are characterized by alack of fiowability whereby in the absence of special mechanicalexpedients the .layer of coating deposited on the paper, as by means ofrolls, is" not smooth but contains irregularitios in depth due to theformation of ripples or ridges. For this reason the art has been muchoccupied with the devising of special means for applying a smoothcoating or for smoothing out the coating after it is applied. The reasonfor the formation of ripples and ridges in the coating when applied, forinstance, by squeeze rolls is due to the property of the coatingcomposition of adhering to both the surface of the roll and the surfaceof the paper; and as the paper leaves the nip of the rolls the coatingis split into two films. The parting of the films takes placeirregularly, so that the coating is applied to the paper in ridges ofvarying thickness. These ridges may vary in width from 0.02 inch .to0.25

inch, depending upon the nature of the coating composition. A number ofmechanical devices such as scraper blades, spinning rolls, brushes orair blasts have been employed to smooth out these ridges. The use ofsuch expedients can 'be avoided only if the film has the property ofleaving the roll completely. This property, closely related to thesyneresis exhibited by certain colloidal solutions, which exude waterreadily, leaving the general mass of the colloidal structuresubstantially unchanged, except for a reduction in volume, is notpossessed in a suificient degree by the ordinary coating compositions.

It is a principal object of my invention to so modify coatingcompositions of standard type as to enable syneresis to occur. This Iaccomplish by incorporating in the coating composition of the typecommonly employed and in which syneresis does not occur, a smallquantity of an easily breakable emulsion of an oleaginous substance ormixture of substances. Such an emulsion, to be suitable, is breakable byrubbing or pressure in the operation of coating, whereby an oily film ismaintained upon the applicator roll, such that on leaving the nip, thecolloidal composition. now in the form of a coating on the paper, tendsto bleed water rather than split the coating film. The water leaving thecoating composition at the point of contact with the oily film in themedium which splits, leaving part of the water on the roll, where it canbe felt by hand and part on the coating. The film of water on the rollprevents the surface of the latter from drying and no deposits ofcoating color form on the roll, so that cleaning devices such as doctorblades or sprays are not required, the roll being quite clean afterseveral hours operation.

In the drawing the single figure, which is selfexplanatory, showsdiagrammatically a pair of squeeze rolls the lower of which is rubbercoated, and a web of paper being passed therebetween and receiving acoating of "color" as above described. The drawing also showsdiagrammatically the occurring syneresis and the superimposed oil andwater films on the applicator roll.-

nauba wax are melted together, whereupon 3 pounds of hot watercontaining 0.015 pound of' caustic. soda are added with thoroughstirring followed by the addition of 50 pounds oi a 4% solution ofstarch. The emulsion so formed is only moderately stable and shows atendency to break and form small agglomerates ofoily material on beingrubbed. The amount of the emulsion so formed can thenbe added to aquantity of a coating composition of a standard type, as for examplegallons of a composition containing 200 pounds of clay, 42 pounds ofcooked starch (such as oxidized starch known in the trade as superfllm,Hercules gum, stayco, etc).

The coating composition so formed may then be colloid and the rollsurface as the paper leaves the nip. More than this will serve no usefulpurpose. Thus a quantity ten times as great as given in the aboveexamplewill result in the formation of a thick layer of waxy material on theapplicator roll, with numerous small lumps floating in the coatingcolor, necessitating frequent cleaning. It is immaterial what type of iaction of the caustic soda.

mineral pigment with which the easily breakable emulsion is employed, i.e., whether clay, chalk, calcium sulfite, titanium oxide, etc., so longas the emulsion is broken at the nip of the squeeze rolls. Thecomposition of the emulsion for the described purpose may be variedconsiderably, as will be evident to those skilled in the art. In theexample above given, sodium stearate is the emulsifying agent, part ofthe stearic'acid having been transformed into sodium stearate by the Theremainder ofthe stearic acid (which has both oleophilic and hydrophilicgroups in the molecule) dissolves in thelubricating oil, a hydrocarboncomprising the internal phase of the emulsion, thereby increasing thestability. A decrease in the amount of stearic acid will thus render theemulsion less stable. The use of diglycol stearate, for example (a knownemulsifying agent), usually results in an emulsion which is not quitestable enough, whereas the use of larger amounts of sodium stearate, orpreferably ammonium stearate and ammonia, would increase the stabilityof the emulsion so that it would not break' in the nip. Furtherstability is also had by the addition of a sodium salt of casein orother protein to the emulsion. A hydrocarbon oil as the chief componentof the internal phase of the emulsion is chosen because it is liquid atroom temperature and readily spreads to form a thin film when theemulsion is broken. The presence of the wax makes the oily film strongerand more stable on the surface of the applicator roll so that pressurewill not readily force it off the surface. It could be omitted, withcorresponding loss in efiiciency. In place of carnauba wax, other waxesor wax-like compounds such as candelilla, bayberry, halow ax, cumar,which are soluble in light lubricating oil, may be used. A nonoil-soluble resin such as copal is not suitable because of its oilinsolubility.

The proper functioning of the emulsion is indicated by the presence ofan oily film on the applicator roll showing an overlie of moisture whichis readily detected by the hand.

The use of the breakable emulsion does not preclude the simultaneous useof emulsions of a more stable kind which may be added to control thegloss and ink reception of the coated paper, so long as these emulsionsdo not break in the nip of the rolls.

The press or squeeze rolls employed may consist of hard or soft rubberor metal or may be a combination of these types, the only requirementbeing that the surface of the applicator roll be such as will enable theoily film to attach itself.

I claim:

1. In the art of coating paper and the like with a coating compositioncomprising an aqueous suspension of mineral pigments and an adhesive bythe use of an applicator roll to which said aqueous. suspension iscontinuously fed and by which said composition is spread out into a filmupon the paper undergoing coating by the aid of a cooperating rollbetween which and said applicator roll the paper is squeezed, the methodof preventing splitting of the applied coating film and adherencethereof to the applicator roll after said coating has been applied tothe paper, which consists in incorporating into said coating compositionan oleaginous emulsion having an oil and water phase and containing ahydrocarbon oil, a.

fatty acid, a soap, and starch, which emulsion is breakable by thephysical rubbing incident to the coating operation above described,whereby in said operation a portion of the oil phase adheres to saidapplicator roll as an oil film thereby causing an adherence to saidapplicator roll of a water film derived from the surface of the coatingmaterial applied to the paper, whereby the coating material is caused toadhere to the paper and not to the applicator roll.

2. A coating composition for coating paper and the like, comprising anaqueous suspension of mineral pigments and an adhesive containing arelatively small quantity of an emulsion in water of an oleaginousmaterial, which emulsion is capable of being broken by rubbing, saidemulsion containing a fatty acid, a hydrocarbon oil, a soap, and starch,

FRANCIS G. RAWLING.

